This invention relates to the recovery of uranium and, more particularly, to a procedure for the calcination of uranium yellowcake produced by precipitation with hydrogen peroxide.
Uranium is produced from uranium bearing ores by various leaching procedures which involve the use of a lixiviant, e.g., an aqueous carbonate-bicarbonate solution or acid solution, to leach the uranium from its accompanying gangue material. The leaching operation may be carried out in conjunction with surface milling operations in which the uranium ore is mined and then crushed and blended prior to leaching, heap leaching of ore piles at the surface of the earth, or in situ leaching in which the lixiviant is introduced into the subterranean ore deposit and recovered therefrom through suitable injection and production systems. Typically, the lixiviant contains a suitable oxidizing agent such as oxygen or hydrogen peroxide which oxidizes the uranium to the hexavalent state at which it forms water soluble uranyl complexes such as uranyl carbonate or uranyl sulfate ions.
The pregnant lixiviant produced during the leaching operation is then processed in order to recover the uranium therefrom. Various procedures may be employed. In one technique, a concentrated uranium solution is produced by passing the pregnant lixiviant over an anionic ion exchange resin and then eluting the resin with an eluant in order to desorb the uranium. The eluting procedure produces a relatively concentrated uranium solution, termed the "eluate", from the ion exchange column. Another technique for concentrating the uranium from the pregnant lixiviant involves solvent extraction. In this case the lixiviant is extracted with a suitable solvent and the solvent then subjected to a stripping action in order to recover the uranium in the relatively rich eluate. Regardless of the concentration procedure used, the eluate is then treated to precipitate uranium to produce the familiar "yellowcake". Various precipitation techniques are described in Merritt, R. C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines Research Institute, (1971), in Chapter 7, "Precipitation and Product Preparation". One suitable technique disclosed in Merritt at pages 247 and 248, involves precipitation with hydrogen peroxide. In this case, the pH of the eluant is adjusted as necessary to an acidic level, e.g. a pH of less than 4, and treated with a stoichiometric excess of hydrogen peroxide. The resulting uranium peroxide precipitates from the eluant to form yellowcake. The uranium product in the yellowcake may be characterized by the formula UO.sub.4.XH.sub.2 O, wherein the value of X normally will range from 2 to 4.
Subsequent to the precipitation step, the resulting yellowcake slurry is separated from the decant and then subjected to additional dewatering steps such as centrifuging or vacuum filtration as described in Meritt at pages 248-251. The yellowcake may then be dried and calcined at an elevated temperature in order to form the uranium oxide final product as described in pages 252-254 of Merritt.